1. Field of the Invention
This invention relates to a display device (panel) using field emission cathodes (hereinafter sometimes referred to as FECs) acting as electron emission sources, (hereinafter sometimes referred to as a field emission display (FED)).
2. Description of the Related Art
When the electric field at a surface of a metal or semiconductor is as large as 10.sup.9 V/m, electrons pass through the potential barrier because of the tunnel effect, thus entering an evacuated space at room temperatures. This phenomenon is called field emission. The cathode emitting electrons utilizing that principle is referred to as a field emission cathode (FEC).
The structure of a field emission cathode called a Spindt type cathode is schematically shown in FIG. 19. Referring to FIG. 19, a cathode electrode 102 of a metal such as aluminum is formed on the cathode substrate 101 such as glass. Cone emitters 105 of a metal such as molybdenum are formed on the cathode electrode 102. An insulating layer 103 such as silicon dioxide (SiO.sub.2) is formed on the remaining portions of the cathode substrate 102 where the emitters 105 are not formed. A gate electrode (or lead-out gate electrode) 104 is formed over the gate insulating layer 104. Openings 106 are formed through the gate electrode 104 and the insulating layer 103. Cone emitters 105 are respectively positioned in the openings 106. The edges of cone emitters 105 are viewed in the openings 106.
The pitch between the cone emitters 105 can be less than 10 .mu.m. Several ten thousand to several hundred thousand emitters can be formed on a single substrate. The distance between the gate electrode 104 and the edge of the cone emitter 105 is set in submicrons. Hence, when a voltage Vg of several 10 volts is applied between the gate electrode 104 and the emitter 105, electrons are field emitted from the emitter 105. When a positive voltage Va is applied to the anode electrode 109 placed so as to confront the gate electrode 104, the anode electrode 109 can collect electrons field-emitted from the emitter 105. In such a condition, a florescent substance coated on the anode 109 which collects electrons field-emitted from the emitter 105 can be glowed. A display device including field emission cathodes can be fabricated by utilizing the above-mentioned principle, This display device is called a field emission display device (panel).
Some high resolution field emission display devices have been proposed that include means for focusing electrons emitted from the emitter of which its locus has a predetermined divergent angle to prevent a leakage of glowed light.
FIG. 20 illustrates the configuration of the above-mentioned field emission display (FED) (refer to Japanese patent Laid-open Publication (Tokkai-Hei) No. 7-104679). In this FED, second gate electrodes (focusing electrodes) 107 are formed for an emitter array corresponding to each pixel formed of plural emitters. Electrons emitted from the emitter array are focused by applying a negative potential to the second gate electrode 107. In FIG. 20, the second gate electrode 107 is formed in a grid pattern so as to surround an array of plural emitters 105. Positive potentials are respectively applied to the anode electrode 109 and the first gate electrode 104 while a negative potential is applied to the second gate electrode 107. The cathode electrode 102 on which plural emitters 105 acting as one pixel, as shown in FIG. 20, are arranged is a unit area. Numeral 111 represents a TFT (thin film transistor) section to drive the cathode electrode 102 in a matrix mode. Electrons emitted from a selected unit area are focused by the second gate electrode 107 and then hit the fluorescent substance 108 formed on the anode 109 with no diffusion.
Japanese patent Laid-open Publication (Tokkay-Hei) No. 6-338274 discloses that the focusing electrode arranged between stripe gate electrodes as well as the adjacent anode electrode are switched at an off level to focus the locus of electrons emitted from an emitter array. FIG. 21 is a diagram used for explaining the above-mentioned field emission display device. FIG. 21(a) is a cross-sectional view showing the field emission display device. FIG. 21(b) is a diagram showing the locus of electrons emitted from an emitter array.
Referring to FIG. 21(a), the cathode electrode 102 is formed in a stripe form on the cathode substrate 101. The gate electrodes 104 in a stripe form are arranged on the cathode substrate 102 through an insulating layer formed on the cathode electrode 102 so as to be perpendicular to the cathode electrode 102. Stripe focusing electrodes 117 are arranged between the stripes of the gate electrode 104. The first anode electrode 118 and the second anode electrodes 119 are in a stripe form and are formed on the anode substrate 110. R fluorescent substance, G fluorescent substance, and B fluorescent substance are sequentially coated on anode electrodes. Numeral 130 represents an anode lead-out electrode A1 connected to each stripe of the first anode electrode 118. Numeral 131 represents an anode lead-out electrode A2 connected to each stripe of the second anode electrode 119. Numeral 134 represents a cathode lead-out electrode derived from each stripe of the cathode electrode 102.
A constant negative voltage is always applied to the stripe focusing electrode 117 via the electrode 135 to focus the locus of electrons emitted from each emitter array 112, as shown in FIG. 21(b). The anode electrodes 118 and 119 are shaped in a stripe form. A voltage of 0 volts is applied to anodes not driven so that a leakage of glowed light can be prevented. In FIG. 21(b), solid lines represent a potential distribution while broken lines represent the electron locus.
FIG. 22 illustrates a field emission display device in which means for focusing an emitted electron beam is prepared for each emitter in a cathode (refer to Japanese Patent Laid-open Publication (Tokkai-Hei) No. 7-29484). In FIG. 22, an insulating layer 103' is additionally laid on the gate electrode (lead-out gate electrode) 104. A focusing electrode (second gate electrode) 107 having a round opening 120 is formed on the insulating layer 103'. That is, the focusing electrode 107 is formed so as to surround the emitter 105. A lower voltage that than to the gate electrode 104 is applied to the focusing electrode 107 so that electrons emitted from the each emitter 105 is focused. Hence the focusing electrode 107 can focus the electrons emitted from the emitter 105.
The focusing electrode 107 traps part of electrons emitted from the emitter 105 and decreases the amount of electrons which reaches the anode electrode, thus increasing ineffective current. The potential of the focusing electrode affects the electric field produced by the first gate electrode, thus decreasing the amount of electrons emitted from the emitter. In order to prevent such problems, the invention disclosed in the prior art publication No. 7-29484, the expression D2=(1.2-2).times.D1 is satisfied, where D1 is the diameter of the opening 106 formed on the lead-out gate electrode 107 and D2 is the diameter of the opening 120 formed on the focusing electrode 107. Thus, electrons emitted from the emitter are focused while the ineffective current flowing into the focusing electrode 107 can be reduced.
The electrons thus emitted reach the anode electrode to glow the fluorescent substance layer coated on the anode electrode. Fluorescent substance dots formed on the anode electrodes in a typical full-color display is illuminated in FIG. 23. In FIG. 23, S1 represents an area corresponding to one pixel, and is, for example, 300 .mu.m in length.times.100 .mu.m in width. S2 represents a fluorescent substance dot which is 220 .mu.m in length.times.80 .mu.m in width.
As described above, the conventional field emission display device is usually driven on a low anode voltage of less than 1 kV. Use of low anode voltage allows the gap between the anode and cathode to be narrowed to 150 .mu.m to 300 .mu.m, thus realizing a very thin display device.
The short distance between the anode and the cathode allows electrons emitted from the emitter to reach the anode with a relatively small divergent width. Hence, the focusing electrode surrounding an emitter array for one pixel as shown in FIG. 20 can focus electrons emitted.
In the high-resolution display, electrons emitted from the emitter array can be focused at the same time by switching adjacent gates and an adjacent anode to an off level, as shown in FIG. 21.
However, in the above-mentioned low-voltage-type field emission display devices, a large anode current (e.g. an anode current density of 50 mA/cm.sup.2 to 100 mA/cm.sup.2) is needed to obtain a predetermined brightness. Generally, the fluorescent substance has a property which shows a low luminous efficiency at large current values.
Recently, field effect display devices which use an anode voltage of more than several thousand kV have been developed to obtain higher brightness at low power consumption. In the high-voltage-type display devices, it is needed that the gap between the anode substrate and the cathode substrate is widened to prevent the cathode-to-anode discharge. This requires means for focusing electrons emitted from the emitter.
Because of the use of a high anode voltage, it is difficult to subject the anode patterned in a stripe form shown in FIG. 21 to a switching operation.
The focusing electrode prepared for each emitter as shown in FIG. 22 does not need the anode switching operation. In this case, there is the disadvantage in that large ineffective current flowing into the first or second gate electrode reduces electrons reaching the anode. That is, the relationship between the size of the opening formed in the first gate electrode and the size of the opening of the second gate electrode is defined in the example shown in FIG. 22. However, since the divergence or diffusion of electrons emitted from the emitter is not considered, the ineffective current flowing into the second gate electrode cannot be sometimes reduced although the emitted electrons can be focused.